Organic light-emitting materials have great commercial potential in a number of areas, including light-emitting devices and displays. Organic materials offer potential advantages in low-cost fabrication, large-area and mechanically flexible devices, and the availability of diverse molecular-structure property relationships.
A conventional polymer electro-luminescent device comprises a thin film of electro-luminescent polymer sandwiched between two electrodes. Polymer electro-luminescent devices are described, for example, in U.S. Pat. Nos. 5,247,190 and 5,399,502 to Friend et al., in U.S. Pat. No. 4,356,429 to Tang, U.S. Pat. No. 4,672,265 to Eguchi et al., U.S. Pat. No. 4,539,507 to VanSlyke and Tang. The entire contents of these patents are incorporated herein by reference.
Efficiency is an important parameter in device design, and is related to the ratio of light energy out to electrical energy in. The quantum efficiency of a device is related to the number of photons emitted relative to the number of charge carriers introduced to the emissive material. Quantum efficiency is in turn dependent on a number of factors, which are discussed in more detail below.
If device efficiency can be increased, brighter displays are possible for the same electrical input. Alternatively, electrical input can be reduced for the same light output, which saves energy and may increase the lifetime of the display, another important design parameter.
Conjugated polymers are often used in organic electro-luminescent devices. These polymers typically comprise a backbone having alternating single and double carbon-carbon bonds, such that extensive electron delocalization occurs. When polymers of this type are used as conducting polymers, an oxidizing agent may be added to remove an electron from a polymer double bond. The remaining lone electron, associated with a positive charge due to the removal of an electron, can then propagate along the polymer chain under the influence of an electric field. This propagating charge is known as a polaron. Reducing agents may be used to donate additional electrons to the chain, which may also propagate along the chains as polarons.
In electroactive devices using conjugated polymers, a polymer film is typically in contact with two electrodes. Electrons are injected into the polymer at one electrode, and electrons are withdrawn from the polymer at the other electrode. The withdrawal of electrons is usually termed hole injection, as the absence of the electron, or hole, propagates in the manner of a positively charged charge carrier. The injected electrons propagate as negative polarons, the injected holes propagate as positive polarons. Electro-luminescence may occur due to the interaction of positive and negative polarons, as discussed below. This interaction may sometimes be termed recombination or annihilation of carriers.
Within the organic layer, charge-transfer (CT) reactions occur between a positively charged polaron (P+) and a negatively charged polaron (P−). The polarons are associated with two participating locations (such as polymer chain segments), and each polaron has spin ½. The interaction between the two oppositely charged polarons leads to the formation of an intermediate encounter complex, involving both locations, and then to the formation of a final state. The final state comprises the ground state of one participant and an excited state of the other participant. The excited state may be either a neutral exciton singlet state (S) or a neutral exciton triplet state (T). Light emission occurs only for singlet exciton decay. The triplet decay channels are non-emissive, an the decay times are typically much longer than the singlet channel decay time. For a light-emissive device, the triplet decay channel is undesirable.
Denoting the formation cross section of the singlet exciton channel as σS and the formation cross section of any one of the three equivalent triplet exciton channels as σT, it is conventionally assumed that the two cross-sections are approximately equal, i.e. the singlet-triplet cross section ratio (σS/σT)˜1. This is true for systems with no significant electron correlation effects. If the two cross-sections are equal, and if there is no energy barrier for the annihilation process of holes and electrons, this results in only 25% of the excited states decaying through the singlet channel, as there will be only one singlet state formed for every three triplet states formed.
The quantum efficiency of an electro-luminescent device is given byη=η1η2η3
where η1 is the singlet emission quantum efficiency, η2 is the fraction of the total number of excitons that are singlets, and η3 is the probability that the injected electrons and holes find each other to form electron-hole pairs. If η2 is limited to 0.25, due to only one singlet state being formed for every 3 triplet states formed, then the maximum electro-luminescence efficiency of the device is also restricted to 0.25. Clearly, it would be of great benefit if this restriction could be eased or eliminated.